Vehicle safety image capture represents a class of video applications characterized by high speed image capture, short event times, and high shock forces. Other applications in this class include safety component testing, and aerospace safety testing. With high speed image capture, frame rates can be less than one millisecond. Moreover, shutter times are even shorter than the frame rate in order to reduce blurring of moving objects in the field of view. Shutter times of less than 300 microseconds are common. In turn, the trend for high speed video is to move from film to charge coupled device (CCD) sensors, and from CCD to complementary metal oxide semiconductor (CMOS) sensors which have less sensitivity. Both short shutter times and reduced sensitivity require very high intensity lighting. It is an object of this invention to provide very high intensity lighting capable of surviving the high shock forces involved in vehicle safety testing.
Another requirement for vehicle safety testing is that equipment must be as light weight as possible for equipment mounted onboard a vehicle during safety testing that involves a vehicle in motion. For high speed image capture that is performed within the cabin of the vehicle, power is generally supplied by onboard batteries, and all necessary equipment to power the lighting, such as power supplies and controllers, must be carried onboard. Therefore, while the efficiency of the lamp technology is important, as measured in lumens per watt, the weight of the equipment required to produce a given value of lumens is the critical ratio. A useful figure of merit in this regard is lumens per kilogram, i.e. the value of lumens that can be produced as a ratio to the weight of equipment needed to power the light (power to weight ratio). It is an object of this invention to provide a ratio of lumens per kilogram greater than 5500.
Current lamp technologies that are used in car safety testing include incandescent and metal halide lamps. Incandescent lamps use a filament that is heated to a high black body color temperature with either an AC or DC electric current. In car crash safety testing, a color temperature of between 5000 and 6000 Kelvin is desirable, with a preferred temperature of 5300 to 5500 Kelvin which is inclusive of the color temperature of the sun. Incandescent technology has numerous drawbacks in car safety testing. Filaments are sensitive to shock and vibration and may fail during high shock forces. In addition, incandescent lamps are very inefficient and produce less than 25 lumens per watt. This also results in low lumens-to-kilogram ratio. Much of the radiation given off by incandescent lamps is in the form of heat (infrared wavelengths), and this is also not desirable in car safety testing. High heat within the onboard systems can affect other equipment adversely as well potentially as cause combustion of materials.
Another technology used in onboard car safety testing is an electrochemical flash lamp, such as used in camera flashes. A major disadvantage of flash lamps is that it captures only very short events of less than a fraction of a second. This limits the temporal window of the event and results in loss of critical data. A related problem is that there is no possibility of electronically checking if the flash lamp is functional. If the lamp fails, then a very expensive test will have been wasted. As such, it is an object of this invention to provide the ability for electronic checking of the viability of the lamp prior to committing the vehicle to a crash or collision. Furthermore, the chemical explosion can cause onboard fires.
Metal halide is another lamp technology that is used in vehicle safety testing. Comments in this specification about Metal halide will also include other members of the family such as high intensity discharge (HID) and hydrargyrum medium-arc iodide (HMI). Metal halide is relatively more efficient than incandescent technology. And Metal halide lamps can be boosted to slightly higher intensity for a limited time. Metal halide can stay at an elevated intensity for longer periods of time than flash lamps. However, the color temperature and CRI (color rendering index) of these lamps changes when the lamps are boosted and over the life of the lamp, so there can be significant lamp to lamp variation in color temperature. This can be particularly problematic where multiple metal halide lamps are used together, as is common in vehicle safety testing. Therefore, it is an object of this invention to provide a lamp with a generally constant color temperature.
In addition, the metal halide lamp is under a high pressure, and this may result in an explosion when under stress. This can be very problematic in vehicle safety testing, particularly for onboard applications. Metal halide lamps require a relatively long warm-up period, up to five minutes, before reaching full operating levels. In addition, these lamps also require a five to ten minute cool down time prior to re-striking. The time required for warm-up, in particular, reduces the efficiency of vehicle safety testing operations. So, it is an object of this invention to provide a lamp with essentially no warm-up period before reaching full operating levels and it is an object of this invention to provide a lamp that does not explode under foreseeable conditions.
Hydrargyrum medium-arc iodide (HMI) is a lamp technology that has as yet found little application in vehicle safety testing. The HMI arc is a concentrated point source, which can produce significant glare and shadows in video images. In addition, the HMI lamp has very prominent intensity in the green region of the electromagnetic spectrum, which can significantly affect color balance in video systems. HMI lamps also change color with age and have AC flicker. Furthermore, HMI lamps can explode under stress. For all of these reasons, HMI lamps, though relatively efficient, have found limited acceptance in car safety testing.
Xenon short arc is another lamp technology that has as yet found little application in car safety testing. Xenon short arc suffers from some of the deficiencies of HMI technology, including glare, shadows from its point source like arc, and extreme vulnerability to violent explosion under stress. Xenon short arc technology has slightly more efficiency than tungsten, generally less than 35 lumens per watt. Xenon, however, does have excellent color temperature properties.
Xenon long arc technology, while previously not used in the car safety testing market, has the color temperature advantages of Xenon short arc, but does not have the glare and shadow effects due to the fact that the arc is quite long (from approximately ten centimeters to meters in length). In addition, Xenon long arc lamps operate under negative pressure, and so do not explode. Xenon long arc lamps also do not use a fragile filament and so are resistant to high shock forces. Xenon lamps are also more efficient than tungsten lamps, producing up to 50 lumens per watt. Xenon lamps also have a very desirable property that they can go from a very low wattage idle to high boost wattage virtually in an instant, and therefore have essentially no warm up time. Furthermore, color temperature remains basically constant throughout the transition from low to high output levels. For the combination of these desirable factors, xenon long arc technology is the lamp technology used in this invention, and an arc length on the order of greater than ten millimeters is preferred.
It is desirable to have a high ratio of high to low illumination level because high speed image capture of onboard vehicle safety testing requires intense light. Typically, frame rates in vehicle safety testing are 1000 frame per second (fps) and shutter times are less than 500 microseconds. Shutter times are shorter than frame times (1000 fps=frame time of 1000 microseconds) because significant blurring of objects can occur during a single frame. Frame times of less than 500 microseconds are more desirable in order to reduce blurring. The only light that is used by the high speed camera is during the open shutter. As shorter and shorter shutter times are used, the light must become proportionately more intense or contrast will be lost. Therefore, in high speed image capture, in order to maintain the same levels of object to background contrast, the average intensity of the light level must vary inversely with shutter time. On the other hand, intense illumination during the time that vehicle safety testing is being set up and sitting idle for preparation is undesirable because equipment may be adversely affected. Hence, a high ratio of high to low output illumination level is desirable.
It is also desirable to have a large depth of focus in high speed image capture. A large depth of focus allows for the imaging of a larger volume of objects. In general, the amount of light entering a camera system varies as the inverse square of the f-number of the camera lens (this is an approximation that is based on the optical arrangement of a point source at a large distance). As the f-number increases, depth of focus is improved (the exact magnitude of this effect can vary according to the type and quality of lens that is used). As depth of focus is improved by increasing the f-number, light levels must go up by the square of the change in f-number. A two fold increase in f-number would approximately require a four fold increase in the high light output level of the illumination system. Once again, this underlines the desirability of a high ratio of high to low output illumination level in vehicle safety testing.
One major supplier of onboard vehicle safety testing equipment is KHS (K. H. Steuernagel Lichttechnik GmbH). The KHS Boost S Light uses a combination of lamp, battery box, and boost box as the onboard system. This system puts out approximately 500 watts of high level output and 125 watts of idle output. The lamp technology used is metal halide, and all of the comments previously made regarding that technology apply to this system. The weight of the system is 29.5 kg (65 lbs) and the luminous flux at high output is 50,000 lumens, for a lumens-to-kilogram ratio of approximately 1700. It is an object of this invention to produce a lumens-to-kilogram ratio of greater than 5000, and preferably greater than 5500. As previously stated, there is a color shift as well as a degradation of the CRI (color rendering index) that occurs as the KHS lamp goes from idle to the high output level, and there is color shift that occurs from lamp to lamp. This is undesirable when using multiple lamps in the onboard car safety test. The ratio of the high output level to the low output level of the KHS light is about 4:1, which results in a heat load at idle of more than 25% of the high output level. Heat load is undesirable in onboard vehicle safety testing because it can affect the performance of instrumented dummies and other equipment in the cabin. Therefore, it is an object of this invention to produce a ratio of the high output level to the low output level of at least about 12:1 and to have an absolute heat load at low output level of less than a metal halide lamp at 125 watts.
The practice of overdriving long arc xenon lamps for short high duration pulses is known. (See, e.g., Pringle, U.S. Pat. No. 5,150,012.) However, in the past this has been accomplished with only relatively high powered lamps with the ignitor active in the circuit to keep the lamp lit during dimming and with only an AC circuit. This has not been done with a medium arc lamp, nor has it been done with the lamp already lit operating at a low idle state, as described herein.